feat: add scanner skeleton and Has repo description rule

[packet-mover]

Summary

• Establishes the project foundation: GitHubClient interface, scanner engine, rule evaluation pattern, and mock-based testing approach
• Implements Rule 1 ("Has repo description"): passes if the GitHub repo description is non-empty, fails if blank or whitespace-only
• Scanner skips archived repos and returns results sorted alphabetically

Files added

File	Purpose
go.mod	Go module definition
client.go	Repo struct + GitHubClient interface
client_mock.go	Mock client for testing
rules.go	Rule definitions (currently: Has repo description)
scanner.go	Scan() — lists repos, evaluates rules, returns results
main.go	Entry point placeholder (reads config from env vars)
rules_test.go	Rule unit tests (pass, fail empty, fail whitespace)
scanner_test.go	Integration tests (skip archived, sort order, error propagation)

Test plan

• go test ./... passes (7 tests)
• Review that the interface shape supports future rules (ListFiles, GetFileContent, etc.)